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Lathes turn to other tasks.

Lathes turn to other tasks

Turning flat on a lathe is nothing new. For decades, single-spindle automatic screw machines (ASMs) could halt spindle rotation, index the workpiece, and then cross drill, mill, broach, etc. More recently, CNC lathes added this capability, while automatics added NC to augment or replace cams.

Amid all this cross pollination, the most common turn-mill feature today, be it option or standard, is some form of live tooling in the turret. Live tools may be self-powered or driven by a power takeoff from the main-spindle drive or other source.

According to D R Jackson of Emco-Maier, Live or rotary tooling on CNC turning centers provides a number of advantages. Foremost is that it often permits completion of a part on a single machine. Live tools provide drilling (including deep-hole, cross hole and bolt-hole pattern), flat and keyway milling, counterboring, slotting, rolling, knurling, sawing, deburring, broaching, and even thread cutting.

A major cost saving is in the purchase price of separate mills, drilling machines, and machining centers that you won't have to buy. Jackson notes, "Adding power tooling to a turning center with a 20 to 30 hp (15 to 22.5 kW) main drive motor will cost an additional $20,000 to $30,000. Compare this with the cost of a CNC milling machine or machining center. And, of course, one piece of equipment takes less floor space than two.

"Along these same lines, the workholding or fixturing expenses for a separate mill are not needed because the same workholding used for turning serves for milling, drilling, tapping, etc. Additionally, you save in direct labor costs--fewer operators and the associated pay rates and fringe benefits."

High quality, low inventory

Often, dimensional integrity is the principal reason for selecting a CNC turning center with power tools. According to Jackson, "Anytime you can reduce the number of workholding elements necessary to hold a component, the final part will be more accurate. If a part must transfer to another machine, its accuracy will likely suffer from build-up of tolerance errors because each fixture in each machine tool has its own tolerance. If we eliminate this gang-up of errors, we minimize the probability of an out-of-tolerance part."

Reduction of in-process inventory is another advantage of live tooling. Jackson says production-control personnel can simplify their jobs by eliminating extra manufacturing steps. For example, they don't have to account for storage of parts between machines, provide extra floor space, or arrange for extra workhandling and transportation.

Jackson says some turning engineers believe they can attain better hole tolerances by rotating the workpiece in the opposite direction of the tool. Certainly, counter rotation can help achieve higher surface speeds required for drilling very small holes. The setup can use lower (available) spindle speeds to get the job done.

Locking up circle patterns

Bolt-hole patterns are often the first tasks considered for a C-axis or power-tool lathe. There are several ways to position the spindle after it is disengaged from the drive motor. One employs fixed degree increments (usually 3 degrees). A second method uses a servomotor drive. Both use a positive lock to hold the spindle for machining after rotation to a known point. These devices work well for simple jobs such as key slotting, milling flats on hex stock, or cross-hole drilling and tapping.

Unfortunately, not everything is that simple. "For jobs such as contouring, spiraling, or miling a helix," says Jackson, "spindle rotation must be coordinated with cutting-tool movement. This requires full C-axis control with a powerful servomotor. Of course, this is more expensive than an incremental degree-indexing system, so budget considerations must be weighed against production needs."

For workpieces such as the one shown at right, some means (manual or automatic) may be required to turn the part around, or transfer it to a subspindle for work on the back side. In some cases, however, a subspindle can eliminate the need for a pickoff device. Daewoo Machine Tools, for example, says the PUMA-8HC can handle some double-ended operations without reversing the workpiece.

Will it clear the work,

do the work?

Serious deliberation is required when selecting a turning center with power-tool capabilities. Workpiece clearances combined with tool interferences must be considered before purchasing a machine. Take a good look at the information published in machine-tool catalogs.

Another concern is total horse-power and speed available to the driven tools. You must have enough power to carry out the tasks at hand. Jackson points out that machines such as the Emcoturn 360 TCM can provide 4500 rpm at 10 hp for hefty milling cuts.

For small parts 10 to 20 mm dia, spindle speeds of 7000 to 8000 rpm (or counter rotation) must be available to optimize tool efficiency. Hardinge offers up to 16,000 rpm. Also, the short cycle times for these parts suggests a drive with minimum acceleration and braking--to cut idle time.

For larger parts (up to 300 mm or 12" turning diameter), typical power ranges from 2.5 to 15 kW at 5000 rpm are not enough. Try to get more.

Tools are mainly driven through a toolholder and adaptor. Some machines provide live-tool power through a ring gear. This design may take greater space, but it doesn't weaken the tool adaptor and can transmit higher torque.

The tool-drive motor is often fitted on the turret slide and synchronized with the main spindle electronically. Synchronized operation is necessary for clean parting off, polygon turning, thread milling, and other special operations. Mechanical synchronization is possible, usually with a direct power takeoff from the spindle motor. But such designs can have more interference with the workpiece and tooling.

How many axes?

Many machines have four controlled axes, including two traveling horizontal turrets and one fixed turret next to the spindle for back machining. Such a machine can finish a workpiece from all sides without additional linear Y-axis control. But, unless there is true Y-axis linear motion, the control can't provide accurate tool-wear compensation during slot milling.

A state-of-the-art

Multifunction machine

The Star MAF-42 Mill/Turn Center introduced by Brown & Sharpe simultaneously performs overlapping functions on multiple axes. In this six-axis design, the C axis is on the fixed headstock; Turrets 1 and 2 both move on X and Z axes; and the subspindle or rear machining unit moves on a B axis parallel to the Z axis. The subturret on Turret 2 holds additional rotating tools.

The six axes provide seven machining motions, and the turrets carry up to 28 tools, which have access to the workpiece from almost every direction. The machine takes bar stock up to 1-5/8" dia, and chucks parts up to 5" dia. The main spindle or C axis employs a 20-hp AC spindle-drive motor providing rotation from 60 to 6000 rpm. The spindle can stop, orient itself, and become a true fifth axis of control.

Turret 1 is a 12-station tool turret with 2-hp drive providing speeds from 150 to 5000 rpm at every other station. Turret 2 is the same, and is independently powered to allow two more axes of control. The subspindle or B-axis synchronizes with the main spindle. It can position itself for secondary operations with the subturret on Turret 2. The subturret is a heavy-duty, rear-end-working station with programmed speeds from 250 to 5000 rpm. A Fanuc 11-TTF computer simultaneously executes two programs independently.

Some machines provide 10 or more axes of control, and of course there are many options to make the turnmill center as complex as desired. Extras range from second turrets, subspindles, and interpolation to robots, automatic loaders, and gantry handling devices for tools and workpieces.

To machine details such as locating faces and holes on the ID, the turning center must have a well-controlled Y axis. Additional linear ways for the turret can provide this mechanically, although it's a design challenge. A sophisticated control and elaborate software can do it, too, by providing linear motion in the X axis and two rotary motions (C axis and turret).

In summary, your use of live tooling can range from simple stations in existing turrets to elaborate turn-mill (or mill-turn) centers coupled with overhead gantry loaders and cell-type machines. For your convenience, we've listed the firms contacted in our random survey. But in reality, almost every lathe builder, large and small, offers some form of live tooling.
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Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Author:Miller, Paul C.
Publication:Tooling & Production
Article Type:Directory
Date:Mar 1, 1989
Words:1388
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